The invention relates to a method for supervising the condition of a transmitter antenna by measuring the magnitude of a signal transmitted to an antenna line and correspondingly the magnitude of a signal reflected from the antenna through the antenna line by producing a measuring signal on the basis of the ratio of the magnitude of the reflected signal to that of the transmitted signal, the measuring signal switching off the transmitter device when it exceeds a first alarm limit of a fixed level.
The invention also relates to a device for supervising the condition of a transmitter antenna, the device comprising a measuring unit for measuring the magnitude of a signal transmitted to the antenna through an antenna line and correspondingly the magnitude of a signal reflected from the antenna through the antenna line, a circuit for producing a measuring signal on the basis of the ratio of the magnitude of the reflected signal to that of the transmitted signal, an alarm indicator, and a first alarm limit of a fixed level.
An essential part of radio systems, such as mobile phone systems and their base stations, is formed by receiver and transmitter antennas, the condition of which has a direct effect on the quality of transmissions in the radio systems. The condition of antennas can be supervised, for instance, by measuring their standing-wave ratio, i.e. the electric matching of the antennas to the rest of the receiving and transmitting system. Conventionally the measuring is carried out by measuring the power supplied to the antenna line and the power reflected from the antenna through the antenna line. The meter has to be a wideband power meter, and thus the power supplied for the measurement has to be high in order for the signals received by the antenna not to interfere with the measurement. However, the use of high power usually causes distortion in electronic circuits, wherefore the measurement result is not reliable. Distortion can be reduced by various filters, but these reduce the accuracy of the measurement result. There are also solutions in which the problems described above have been solved by using as a measuring signal a measuring frequency outside the frequency band of the system and by using narrowband power measurement specifically matched to the measuring frequency.
European Patent Application No. 261 828 discloses an apparatus which measures a signal supplied from a measuring source directly to a network and a signal reflected from the network under analysis for determining the relative power. The way the apparatus analyzes the reflected and the transmitted signal is, however, a problem. For example, a sample signal extracted from the output power is applied to a measuring device over a path which is different from that of a sample signal extracted from the power reflected from the network under analysis. The fact that the signals propagate over different paths to the measuring device renders the relative value of the signals inaccurate. The inaccuracy of the relative value is caused by imperfections in the components on the signal path and inaccuracies formed during manufacture. For measurement, the measuring path must therefore always be calibrated separately by a standard signal. In practice, this means that each combination of a measuring device and an antenna with its cables would have to be calibrated by a standard signal in connection with the manufacturing process.